It is known in the field of polymer science that interpenetrating polymer networks (IPNs) are blends or alloys of two or more polymer components, each of which is a crosslinked three-dimensional network. The individual polymer component networks are more or less physically entangled with, but not covalently bonded to the other polymer network(s) in the IPN. The structure of an IPN is frozen by physical interlocking between the component polymer networks [Sperling1997].
A feature of IPNs is that they permit combining advantageous properties from each of two polymers which are normally incompatible. For example, in a hydrophobic-hydrophilic system, flexibility and structural integrity might be imparted by the hydrophobic polymer and lubriciousness might be imparted by the hydrophilic polymer. An IPN may be a bicontinuous system in which each of the polymers form a continuous matrix throughout the network.
Two methods for making IPNs are the sequential polymerization method, and the simultaneous polymerization method. In a typical sequential polymerization method, a solid polymer film (host) is swollen with a monomer (guest) mixture containing an initiator and a crosslinker to form an IPN reaction mixture which is then "cast", or placed, against a solid surface or suspended in a gaseous mixture such as air or nitrogen, and initiated, for example with heat or UV radiation, to initiate polymerization and crosslinking of the monomer(s). Due to the guest monomer's low molecular weight, it can be distributed readily throughout the host polymer. Polymerization and crosslinking of the guest monomer to form a polymer network then results in an entangled polymer network of a first polymer (the host) and a second polymer (derived from the guest).
In a typical simultaneous polymerization method of making IPNs, monomers or prepolymers are mixed along with a polymerization initiator and crosslinking agent of both networks, to form an IPN reaction mixture which is then "cast", or placed, against a solid surface or in contact with a gaseous mixture such as air or nitrogen. Polymerization and crosslinking of the component monomers may occur simultaneously, but by non-interfering reactions, to form an IPN composed of covalently independent but physically entangled component polymeric networks of a first polymer (derived from a first monomer or prepolymer) and a second polymer (derived from a second monomer or prepolymer).
Examples of typical methods of preparation of hydrophobic-hydrophilic IPNs using sequential and simultaneous methods can be found in U.S. Pat. Nos. 5,424,375 and 4,752,624 respectively. In both cases the starting mixtures were cast against glass or solid substrates that were used as molds. Substrates were chosen for their inertness and ease of demolding.
Over the years, variations in the methods used to prepare sequential and simultaneous IPNs have produced materials of widely different properties and morphologies. These include latex IPNs, in which spherical particles having a core-shell structure are formed using emulsion polymerization techniques. The original seed latex of a crosslinked polymer is immersed in a solution of a monomer, together with crosslinker and activator. Also there are gradient IPNs, in which the overall composition within the material varies from location to location on a macroscopic level. Gradient IPNs are prepared by immersing a polymer network in a solution of monomer or prepolymer. Polymerization and crosslinking of the monomer takes place as it diffuses into the host polymer network. A further type are thermoplastic IPNs, also known as polymer blend-IPN hybrids, which are prepared with physical crosslinks rather than chemical crosslinks. These IPNs flow at elevated temperatures, but at room temperature they are crosslinked and behave like IPNs. Yet another type of IPN is the semi-IPN which can be prepared by any of the above methods, in which one or more polymers are crosslinked and one or more polymers remain linear.
U.S. Pat. No. 4,423,099 concerns the preparation of gradient IPNs in which a hydrogel is swollen with water and reactant and then immersed in a medium containing a co-reactant. As the co-reactant is diffused into the host polymer network, an interpenetrated polycondensation polymer is formed within the hydrogel network. The compositional gradient of the polycondensation polymer varies from a high concentration at the surface to zero within the bulk.
U.S. Pat. No. 5,183,859 describes a latex IPN structure formed by a consecutive mutli-stage emulsion polymerization process. A rubbery polymer formed in an earlier stage is covered with a hydrophilic polymer formed in a later stage. The resultant polymer particle comprises a rubbery polymer core and a methacrylic glassy polymer shell.
U.S. Pat. Nos. 4,423,099 and 5,183,859 describe processes carried out to achieve a gradient composition within the IPN.
The methods used in the preparation of gradient IPNs and latex IPNs involve the step of immersion of a polymer network in a solution containing a monomer or prepolymer. This step is for the transfer of monomer/prepolymer into the IPN, and gives a gradient profile ranging from 100% of one polymer component at the surface to 100% of the other polymer component in the centre of the IPN. In the preparation of latex IPNs, monomers or reactants in the immersion solution become covalently linked components of the IPN. In the preparation of a gradient IPN the chemical potentials of the polymerizable reactants in the immersion solution are not the same as the chemical potentials of the same reactants in the reaction mixture. Such IPNs do not have a bicontinuous morphology throughout the IPN.
It has been postulated that there are two distinct mechanisms of phase separation during IPN formation: spinodal decomposition (SD), which form bicontinuous structures of relatively small, interconnected nodular domains of the guest polymer in the host polymer; and nucleation and growth (NG) which forms isolated guest polymer domains (or islands) dispersed in a continuous host polymer phase (sea), also referred to as sea-island morphology. The guest monomer concentration is believed to determine which occurs. In one report of a particular IPN system, it was found that IPNs with guest monomer concentrations of greater than 20% produced a bicontinuous morphology in the bulk of the completed IPN. This very stable morphology characterized by bicontinuity and very small domains is highly desirable for applications such as impact resistance polymers, reinforced elastomers, sound and vibration damping, rubber blend and electrical insulators.
The term "Bicontinuous morphology", generally refers to at least two regions, each of substantially uniform composition which differs from the other, and each of which forms a continuous pathway from one surface of an article to another surface of an article. Thus an IPN having a bicontinuous morphology of hydrophilic and hydrophobic polymers will have two continuous pathways or two sets of continuous pathways extending from one surface of the IPN material to the other surface.
Surprisingly, it has now been reported that even so-called bicontinuous IPNs do not have a bicontinuous morphology at the surface. Murayama [Murayama1993, Murayama1993a] et al prepared hydrophobic and hydrophilic IPNs and concluded that a thin layer of one polymer existed at the surface.
Lipatov [Lipatov1999] suggested that the structure and composition of the surface layers of an IPN which had been formed near the interface with a solid substrate were dependent on the surface energy of that solid substrate.
In other work, it was reported that polyurethane-polystyrene IPNs which had been prepared by a method that included a step of casting the IPN reaction mixture against glass sheets showed a sea-island morphology, and had a surface that was enriched in the polyurethane component which was attributed to the nature of the casting substrate [Rohl 1999].
U.S. Pat. No. 5,424,375, Process for the Manufacture of a Polymeric Material containing Interpenetrated Polysiloxane-Polyacrylic Networks, also reports surface segregation wherein the surface properties were dominated by the polysiloxane component.
U.S. Pat. No. 4,931,287 describes the use of hydrophobic-hydrophilic IPNs to provide a controlled release drug matrix having a sea-island morphology (i.e. not bicontinuous). It was found that delivery of hydrophobic solutes from a continuous hydrophobic matrix with a discrete hydrophilic phase occurred from localized surface regions and then decreased to zero due to a diffusional barrier created by the hydrophobic polymer, even though a significant amount of the drug was still in the matrix. Thus in this case, the lack of bicontinuity was a major drawback of this material IPNs composed of multiple hydrophilic components, at least one of which is stimuli-responsive, have also been reported. A primary goal in these applications has been to improve the wet strength of the responsive hydrogel component. For example, pH-responsive IPNs of poly(vinyl alcohol) and poly(acrylic acid) have been prepared for applications such as drug and protein separation processes, drug delivery systems and pervaporation membranes. Complexation between the two IPN polymer components by hydrogen bonding has also been manipulated via pH and thermal stimuli to produce variable permeability membranes.
U.S. Pat. No. 5,580,929 discloses preparation of an IPN from two gel components, in which the gel underwent a large volume change in response to physical and/or chemical stimulus for use in drug delivery applications.
It may also be advantageous to use hydrophilic-hydrophobic IPN materials for stimuli-responsive applications to provide stronger materials and better control of pulsatile drug release profiles.
Although hydrophilic-hydrophobic IPN materials show great promise in the drug delivery and biomedical fields due to their inherent stability, bicontinuous morphology and small domain size, their lack of bicontinuity at the surface has been a major drawback to their use in a variety of applications. Continuity in the permeable phase is necessary for drug delivery applications, while bicontinuity is required for applications such as contact lens materials and wound healing dressings, where transport of oxygen and water occur in different phases. It has also been reported that a hydrophilic-hydrophobic microdomain structure shows great promise as a blood compatible material[Kataoka1985]. Hydrophilic surfaces of strong elastomeric materials is also a prerequisite for catheter materials.
Conventionally, for the preparation of simultaneous and sequential IPNs, to effect polymerization and crosslinking, the pre-IPN is typically removed from the guest monomer solution and cast against a solid substrate and polymerization and crosslinking is then effected. This procedure affects the surface morphology of the IPN. At the interface between the solid substrate and the pre-IPN surface, surface thermodynamics may result in a surface concentration of one component that is higher than, equal to, or lower than the concentration of that component in the bulk of the IPN. For example, if a hydrophilic guest monomer such as methacrylic acid (MAA) is distributed in polydimethylsiloxane (PDMS), and placed in contact with a hydrophilic substrate such as glass, the higher affinity of glass for MAA relative to PDMS, will result in a preferential enrichment of MAA at the surface--even when equilibrium is reached. Thus, the nature of the casting substrate affects the guest monomer concentration distribution in the pre-IPN, particularly in the surface region of the pre-IPN. Such effects are pronounced in the case of two very different, incompatible compounds such as a hydrophobic polymer/monomer and a hydrophilic polymer/monomer, since the surface energies of the two compounds are very dissimilar.
Furthermore, in the conventional methods for the preparation of sequential IPNs, to effect polymerization and crosslinking, the pre-IPN or reaction mixture is typically removed from the guest monomer solution and cast against a solid substrate and polymerization and cure is then effected. This step exposes the pre-IPN for an uncontrolled amount of time to an environment which contains no monomer. Evaporation of monomer during this step may result in a significant decrease in the concentration of the monomer at and near the surface of the IPN.
It is an object of the present invention to provide an improved method of making IPNs.
It is a further object of the present invention to provide a method of making IPNs which permits improving the surface morphology of the IPN.
It is another object of the present invention to provide an improved IPN product.